Transport refrigeration units (in singular, “TRU”) have been used for many years in semi-trailers for hauling goods that must be kept refrigerated, such as food products susceptible to spoilage or melting. The power needed for driving a full size TRU is about 30 horsepower.
Refrigeration systems generally have three basic components for performing a refrigeration cooling cycle: a compressor for raising the pressure of a vaporized refrigerant; a condenser for allowing the compressed vaporized refrigerant to condense to a liquid form and thereby to absorb heat from a volume to be cooled; and an evaporator for allowing the condensed and now warmed refrigerate to evaporate and thereby to give off the absorbed heat to the environment. The evaporated refrigerant is then returned to the compressor from the condenser to repeat the cycle. The compressor is driven by an engine or motor to provide the power needed for the cooling cycle.
Some small trucks have cargo refrigeration units where the compressor is mounted in the truck engine bay and turned by a belt wrapped around a pulley take off from the crankshaft of the truck engine. Essentially the same system configuration is used in passenger automobile air conditioning systems.
U.S. Pat. No. 7,086,241 to Holt describes a hydraulic power unit for a refrigeration system that would typically be used in a diesel truck tractor and semi-trailer combination that drives the compressor with a hydraulic motor, the hydraulic motor being driven in turn by hydraulic fluid circulated by a hydraulic pump turned by the truck engine.
Many TRU's employ an electric motor for turning the compressor, where the electricity that is needed for powering the electric motor is generated by a small, on-board diesel engine and generator combination. Such a diesel TRU is shown in U.S. Pat. No. 8,776,928.
The '928 Patent describes a regenerative electric drive methodology that also employs a hydraulic pump for powering a diesel TRU. The hydraulic pump is driven by a differential of an axle of the truck and drives a remotely located hydraulic motor and generator combination for generating electricity that can be switched in to the TRU as an alternate source of electric power for the TRU when the truck engine is available to produce this power and the dedicated diesel engine in the TRU is therefore not needed.